Azimuthal resistivity measurement while drilling has been widely used for tracking bed boundaries during geosteering operations. The information about the direction of and distance to a bed boundary is important in landing a well in right spots (e.g., pay zones). The measurement may also be used for resolving formation resistivity anisotropy properties for reservoir formation evaluation.
One key component in azimuthal resistivity measurement is azimuthal antennas. It is different from a non-azimuthal antenna in that the radiation pattern of the former will change as the tool rotates, whereas that of the latter does not. Two major types of azimuthal antennas have been used in making azimuthal resistivity measurements. The first is a transversal antenna which has its direction of magnetic moment normal to the longitudinal axis of a tool. Examples include those taught by Wang in U.S. Pat. No. 9,268,053. The second is a tilted antenna with its magnetic moment oriented at an angle from the longitudinal axis of a tool. In either way, the azimuthal antenna will be centered on the tool axis. Depending on the type of azimuthal antennas used, the voltage signals acquired may be used in different ways to derive information about bed boundaries for geosteering applications. For instance, with transversal antennas, the voltage signals (in-phase, out-of-phase or both) may be processed directly for bed boundary information. For tilted antennas, however, the voltage signals must first be processed to remove non-azimuthal component. This usually done with the help of tool rotation because the non-azimuthal component usually does not depend on tool face angle and may be removed.
One serious challenge to any azimuthal resistivity measurement is temperature effects. At higher temperatures, antennas and associated electronics will change their characteristics with temperature. The measured signals or derived quantities will then display temperature dependent variations. More than often, the variations may severely distort, or even mask, the information about the surrounding formations. Therefore, any azimuthal resistivity measurement should be compensated for temperature effect. In conventional (propagation) resistivity measurement, this is done by employing a dual-receiver and dual-transmitter antenna configuration. For the method to work, two receiver antennas are placed in between two transmitter antennas. The two receiver antennas are spaced apart in the longitudinal direction by, e.g., a few inches. The same couple of receiver antennas are used to measure signal attenuation and phase difference responses for both transmitter antennas (fired sequentially). Taking average of the attenuations or phase differences will largely remove temperature effects.
Applying the same principle to azimuthal resistivity measurement would similarly require at least two azimuthal receiver antennas to be spaced apart along the tool axis. Because azimuthal receiver antennas usually are employed together with non-azimuthal receiver antennas to form a complete resistivity measurement, spacing apart the azimuthal receiver antennas would either substantially increase the tool length or become practically difficult to implement.
It is possible to co-locate two azimuthal receiver antennas to eliminate the requirement for additional tool length. However, the close proximity of the antennas will inevitably cause signal interference between them, thus reducing the sensitivity of the measured azimuthal signals to adjacent boundaries. For instance, two tilted antennas as shown in FIG. 1 will leave their windings electromagnetically exposed to each other, causing the antennas picking up each other's signal. Thus, there are needs to improve the azimuthal measurements with different directional antennas.